Honestly. Not all teenage boys are into girls, and cars. And the only reason I wanted a summer job as a high schooler was to pay for my aquariums. We have much more going on under the surface than meets the eye.
Pro-college- I'm nearly 100% certain that my failure to keep a yellow tang in a 10 gallon aquarium when I was 10 (and my subsequent essay about it), was able to get me into some of America's top colleges like Stanford+the Ivies, as well as the waitlists of Ivies that didn't accept me at first. So yeah, my hobby, and the interest in marine biology that occurred as a side note, got me into a bleeping great university.
Con-Time sucker- As a high schooler, balancing four aquariums with varsity swim, first chair in marching and concert band, #1 rank in the school, and community service, was hard. Aquariums often took a back seat to competitions and grades throughout the school year. So it's not going to look perfect for a few weeks of the year.
Pro-Always stuff to do- In regards to the above con, I was never bored though. This forum, books, magazines, and the aquariums themselves were more than enough to keep me always busy.
Con-Nobody's going to know what the heck you're doing- Believe me. I tried teaching my parents about the nitrogen cycle for 8 years straight...they still haven't read the required chapters that explain why I change "clean" water for newly made water. The parents might find it pointless.
Pro-Potential entrepreneurship opportunity- If he has the time, your friend could look into breeding/raising marine life in terms of fragging sessile inverts like coral, or even breeding simpler fish like clownfish and banggai cardinalfish. I never did this, as I was never able to locate good broodstock, but there's a pair of banggai's waiting for me to come back home for the summer...
Con-Expensive-There's no going around this. Coral's expensive unless you go to the $5 frag rack. The cheapest fish are 7$ damselfish, while most other fish are $30+ for the nicer ones. The tank and equipment will make him go into sticker shock. A modest 20H (what I would recommend to past me) +sump and everything else could easily go into 300-400 $ depending on equipment. Good thing there are internet sales to counteract all of this.
Pro-Intro to Marine Biology- Again referencing the yellow tang getting me into college, I can say that it really got me into marine biology. While I can't start taking classes on marine bio until next year due to prerequisites, I look forward to classes covering coral reef biomes, amidst other things. I also got a leg up on applying to marine biology internships due to my experience with marine organisms.
Con-Doesn't really help with marine biology as a study- I learned this while taking a summer class on marine biology. Reefkeepers don't really pay attention to thermoclines and gyres as well as how they affect biology etc. Aquariums are a good place to get a good look at what's overlooked on a reef, but they don't really agree with what most professors study and internships offered in my case.